The outlook on Britain's triple-A rating remains negative due to worries over the government's rising debt burden, ratings agency Standard & Poor's said on Monday.

S&P said it expected to review the rating once the medium-term fiscal outlook became clearer after the election, expected on May 6.

It suggested it would have enough information to make this judgment by the end of 2010 whoever wins the election as both main political parties have promised a spending review by then.

"We believe that substantial uncertainty persists with regard to the details of what the current government has indicated will be a largely expenditure-focused fiscal consolidation program starting next year," S&P said.

"We believe that additional spending measures will likely be required to put the public debt burden on a clear downward trajectory later in the current decade."

S&P cut the outlook on Britain's AAA rating to "negative" on May 21, 2009, a move that typically carries a one in three chance of a downgrade.

Both Moody's and Fitch have a triple A ratings on Britain with a stable outlook.